Self-propelled or powered wheelchairs have vastly improved the mobility/transportability of the disabled and/or handicapped. Whereas in the past disabled/handicapped individuals were nearly entirely reliant upon the assistance of others for transportation, the Americans with Disabilities Act (ADA) of June 1990 has effected sweeping changes to provide equal access and freedom of movement/mobility for disabled individuals. Notably, various structural changes have been mandated to the construction of homes, offices, entrances, sidewalks, and even parkway/river crossings, e.g., bridges, to include enlarged entrances, powered doorways, entrance ramps, curb ramps, etc., to ease mobility for disabled persons in and around society.
Along with these societal changes, it has become possible to offer better, more agile, longer-running and/or more stable powered wheelchairs to take full advantage of the new freedoms imbued by the ADA. More specifically, various technologies, initially developed for the automobile and aircraft industries, are being successfully applied to powered wheelchairs to enhance the ease of control, improve stability, and/or reduce wheelchair weight and bulk. For example, sidearm controllers, i.e., multi-axis joysticks, employed in high technology VTOL and fighter aircraft, are being utilized for controlling the speed and direction of powered wheelchairs. Innovations made in the design of automobile suspension systems, e.g., active suspension systems, which vary spring stiffness to vary ride efficacy, have also been adapted to wheelchairs to improve and stabilize powered wheelchairs. Other examples include the use of high-strength fiber reinforced composites, e.g. graphite, fiberglass, etc. to improve the strength of the wheelchair frame while reducing weight and bulk.
One particular system which has gained widespread popularity/acceptance is the mid-wheel drive powered wheelchair, and more particularly such powered wheelchairs with anti-tip systems. Mid-wheel drive powered wheelchairs generally have a pair of drive wheels with a common rotational axis positioned slightly forward of the combined center of gravity of the occupant and wheelchair to provide enhanced mobility and maneuverability. Anti-tip systems provide enhanced stability of the wheelchair about its pitch axis and, in some of the more sophisticated anti-tip designs, improve the obstacle or curb-climbing ability of the wheelchair. Such mid-wheel powered wheelchairs and/or powered wheelchairs having anti-tip systems are disclosed in Schaffner et al. U.S. Pat. Nos. 5,944,131 & 6,129,165, both issued and assigned to Pride Mobility Products Corporation located in Exeter, Pa.
While such wheelchair designs have vastly improved the capability and stability of powered wheelchairs, designers thereof are continually being challenged to examine and improve wheelchair design and construction. For example, the Schaffner '131 patent discloses a mid-wheel drive wheelchair having a passive anti-tip system. That passive anti-tip system functions principally to prevent forward tipping of the wheelchair. The anti-tip wheel in the Schaffner '131 patent is pivotally mounted to a vertical frame support about a pivot point which lies above the rotational axis of the anti-tip wheel. Because of the geometry of the passive anti-tip system, the anti-tip wheel must contact a curb or other obstacle at a point below its rotational axis to cause the wheel to “kick” upwardly and climb over the obstacle. Consequently, this geometric relationship limits the curb-climbing ability of the wheelchair.
The Schaffner '165 patent discloses a mid-wheel drive powered wheelchair having an anti-tip system which is “active” in contrast to the passive system discussed previously and disclosed in the '131 patent. That active anti-tip system is responsive to torque applied by the drive motor, or pitch motion of the wheelchair frame about its effective pitch axis, to vary the position of the anti-tip wheels actively, thereby improving the wheelchair's ability to climb curbs or overcome obstacles. More specifically, the active anti-tip system of the Schaffner '165 patent mechanically couples the suspension system of the anti-tip wheel to the drive train assembly such that the anti-tip wheels displace upwardly or downwardly as a function of the magnitude of: (i) torque applied by the drive train assembly, (ii) angular acceleration of the frame or (iii) pitch motion of the frame relative to the drive wheels.
FIG. 1 is a schematic view of a power wheelchair with an active anti-tip system 110 similar to that disclosed in the Schaffner ′165 patent. The drive train and suspension systems shown in FIG. 1 are mechanically coupled by a longitudinal suspension arm 124, pivotally mounted to the main structural frame 103 about a pivot point 108. A drive train assembly 107 is mounted at one end of the suspension arm 110, and an anti-tip wheel 116 is mounted at the other end, at the front of the wheelchair. In operation, torque from a drive wheel 106 is reacted by the main structural frame 103, resulting in relative rotational displacement between the drive train assembly 107 and the frame 103. The relative motion therebetween, in turn, effects rotation of the suspension arm 124 about its pivot axis 108 in a clockwise or counterclockwise direction depending upon the direction of the applied torque. That is, upon a forward acceleration, or increased torque input (as may be required to overcome or climb an obstacle), counterclockwise rotation of the drive train assembly 107 as seen in FIG. 1 (from the side of the wheelchair that is to the user's right) will occur, effecting upward displacement of the anti-tip wheel 116. Consequently, the anti-tip wheels 116 are “actively” lifted or raised to facilitate operational modes such as curb climbing. Alternatively, deceleration causes a clockwise rotation of the drive train assembly 107 as seen in FIG. 1, thus effecting a downward displacement of the respective anti-tip wheel 116. The downward motion of the anti-tip wheel 116 also assists to stabilize the wheelchair when going down a slope. Here again, the anti-tip system “actively” responds to a change in applied torque to vary the position of the anti-tip wheel 116.
While the active anti-tip system disclosed in the Schaffner patent '165 offers significant advances by comparison to prior art passive systems, the one piece construction of the suspension arm 124, with its single pivot connection 108, necessarily requires that both the drive train assembly 107 and the anti-tip wheel 116 move through the same angle about the pivot 108, relative to the frame 103. As a result, the arc length or up or down displacement of the anti-tip wheel 116 is limited by the angle through which the drive train assembly 107 moves. The single pivot mount design, while elegant and simple, thus limits the freedom available for the designer to satisfy other requirements.
Moreover, when the anti-tip wheel 116 contacts a vertical curb or obstacle at or near a point which is in-line with the wheel's rotational axis, the point of contact is below the pivot connection 108. That will produce a force couple rotating the suspension arm 124 downwardly, so the anti-tip wheel 116 will also tend to move downwardly. This downward travel is, of course, contrary to a desired upward motion for climbing curbs and/or other obstacles.
Other wheelchair anti-tip systems exist, such as the one illustrated and described in published International Patent Application No. WO 03/030800 A1 assigned to Invacare Corporation. This suspension/anti-tip system employs an arrangement of links. The anti tip wheel moves up and down because the anti tip wheel is mounted on the front end of a fore-and-aft suspension arm carrying the motors and drive wheels. In addition, the anti tip wheel swings rearwardly and upwardly about the front end of the suspension arm when the front end of the suspension arm rises, and vice versa.